fix(punktfunk/1): adversarial-review fixes — SPAKE2 pairing, renegotiation hardening, +more

Triaged the multi-agent review of the renegotiation + pairing + Sway + AV1/surround batch
(1 critical, 11 major/minor confirmed). Fixes:

CRITICAL — PIN pairing was offline-brute-forceable. The HMAC-of-PIN proof let an active
MITM who terminates the TOFU ceremony recover the 4-digit PIN by offline dictionary search
(all other inputs observable) and forge a correctly-bound proof. Replaced with **SPAKE2**
(balanced PAKE, `spake2` crate) + key-confirmation MACs, binding both cert fingerprints as
the SPAKE2 identities: an attacker gets exactly ONE online guess, no offline search, and
mismatched cert views (a real MITM) never reach a shared key. Also reworked the UX to an
"arming PIN" — one PIN per arming window shown at host startup (the SPAKE2 client needs the
PIN to build its first message, so it can't be minted per-connection). Validated live:
wrong PIN rejected in 0.1s, right PIN pairs + persists + the paired identity streams.

Pairing hardening: `--allow-pairing`/`--require-pairing` must arm pairing (default rejects
unsolicited ceremonies); per-host cooldown bounds online guessing; the client flushes its
CONNECTION_CLOSE so a refused ceremony can't wedge the sequential host for the full timeout;
atomic (temp+rename) paired-store writes.

Protocol: control/pairing messages use a distinct CTL_MAGIC (PKFc) — fully disjoint from
the positional Hello namespace (a future abi_version can't be misparsed as a control
message); all typed decodes are length-exact. ABI_VERSION → 2 (punktfunk_connect signature
gained the identity params; header regenerated).

Renegotiation: drain the reconfig channel to the NEWEST mode (one rebuild, not one per
stale step); validate refresh_hz; build the new pipeline BEFORE dropping the old so a
rebuild failure keeps the session on its current mode instead of killing it.

GameStream: packetDuration snaps to {5,10} (an in-between value isn't a legal Opus frame
size and would kill audio). Sway: chooser file moved to $XDG_RUNTIME_DIR (was a fixed
world-writable /tmp path — DoS / capture-misdirection by another local user).

Swift: fixed two compile breakers in the new pairing/identity APIs (Int32 status .rawValue,
UInt cap cast). New SPAKE2 + namespace-disjointness + pairing-roundtrip unit tests; the
in-process pairing test now also exercises the arming PIN + cooldown. 114 tests green,
clippy -D warnings clean (both feature sets), fmt, C-ABI harness.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

clients/apple/README.md

@@ -117,11 +117,13 @@ signing, bundle id `io.unom.punktfunk`. Notes:
    contract documented on the constructors; the host accumulates them into a virtual
    Xbox 360 pad). Poll `nextRumble()` and feed `GCDeviceHaptics` for force feedback.
    Client-side capture isn't in `InputCapture` yet.
-7. **Trust — the full ceremony exists now.** `generateIdentity()` once (persist both
-   PEMs in the Keychain), then `pair(host:identity:pin:name:)` with the 4-digit PIN the
-   host displays (its log; UI later) — returns the host's VERIFIED fingerprint; persist
-   it and pass `pinSHA256:` + `identity:` to every connect. A wrong-size pin throws
-   `.invalidPin`, a wrong PIN `.wrongPIN`. The TOFU flow `PunktfunkClient` already
+7. **Trust — the full ceremony exists now (SPAKE2).** `generateIdentity()` once (persist
+   both PEMs in the Keychain), then `pair(host:identity:pin:name:)` with the 4-digit PIN
+   the host prints when it ARMS pairing (`--allow-pairing`/`--require-pairing`; one PIN
+   per arming window, shown at startup — the user reads it before pairing). Returns the
+   host's VERIFIED fingerprint; persist it and pass `pinSHA256:` + `identity:` to every
+   connect. Pairing is a real PAKE: a wrong PIN gets ONE online guess (no offline
+   dictionary attack), throwing `.wrongPIN`; a wrong-size pin throws `.invalidPin`. The TOFU flow `PunktfunkClient` already
    implements (fingerprint confirmation sheet, per-host `HostStore`, "Forget Identity")
    keeps working against hosts not running `--require-pairing`; upgrading the sheet to a
    PIN-entry field closes the remaining gap — with `--require-pairing` the host now